Efficient generation, collection and transmission of ions with minimum loss upon ejection of charged droplets from an ion source are of paramount importance to increase sensitivity and minimize the amount of sample required for stable mass spectrometry analysis. It is accepted that the ion production efficiency from the moment the sample solution is sprayed until it reaches the medium-vacuum regions of the MS is barely in the 0.01-0.1% range, if no special transmission enhancement methods are used. This problem has been recognized as early as in late 1980's when Henion and co-workers proposed to improve the spray formation process by using a sheath flow of nebulizing gas to enhance aerosolization. Later, Covey et al. further improved the charged droplet desolvation process with the addition of external flow of heated gas that was directed towards the nebulizer-assisted electrospray. This gas lowered the solvent load into the first MS stage and improved desolvation, translating into a 10-fold increase in sensitivity. Both of these approaches are now commonly used in modern ESI ion sources.
A more recent approach to increase ESI sensitivity focuses on improving charged droplet collection efficiency by means of electrohydrodynamic focusing of the ESI ion beam. This approach, first developed by Shaffer et al., utilized a hybrid RF-DC “ion-funnel” device, which produced an average 10-fold improvement in sensitivity. Further refinements to this approach involved the use of a multicapillary inlet and a jet disruption device placed in the 1-2 Torr region of the atmospheric pressure MS interface. Recently, Zhou et al. and Hawkridge et al. demonstrated the use of an industrial air amplifier based on the Venturi and Coanda effects to focus charged electrospray droplets resulting in an 18-fold increase in signal intensity (when a potential bias was applied to the amplifier) as well as a 34-fold reduction in the detection limit. Despite all these advances, the design and operation of droplet/ion transmission interface are far from being optimal.